Manufacture of insulated electric conductors

ABSTRACT

A mixture of substances is fed to an extruder and, after being homogenized and liquefied, is extruded on an electric conductor or as a pipe. To meet electric cable specifications, the insulating material used is a polyolefin, such as polyethylene, cross-linking of which is brought about by the addition of a silane, a free-radical generator, and a silanol condensation catalyst. After extrusion, cross-linking takes place in the presence of moisture. To facilitate penetration of the moisture and accelerate cross-linking, an agent capable of reducing the rate of crystallization of the polymer, such as vinyl acetate, is used as a further additive.

This is a continuation of application Ser. No. 134,634 filed Mar. 27,1980, now U.S. Pat. No. 4,351,790 issued Sept. 28, 1972.

This invention relates to insulated electric conductors, moreparticularly to such conductors of the type having an insulatingcovering of cross-linked plastic material, and to a method for theirmanufacture.

During the past few years, there has been a rapid development in thetechnology for manufacturing insulated electric cable intended toconduct current at high or low voltages. This development is likewisenoted in the manufacture of certain kinds of pipes and tubing made ofsynthetic materials analogous to those used for forming the insulatingsheaths of cable.

German Disclosed Application (DOS) No. 23 50876 teaches the reaction ofcertain polymers, such as ethylene and vinyl acetate copolymers, with asilane in the presence of a free-radical generator, then exposing thisproduct, containing various additives as well, to water in order tocause cross-linking of the polymer. These materials can be shaped byextrusion, followed by a treatment in the presence of either boilingwater or water at a temperature between 50° and 80° C.

French Published Application No. 2,217,362 and German DisclosedApplication (DOS) Nos. 24 19209 and 24 19210 also relate to methods andproducts of the same type.

According to U.K. Published Application No. 2,002,395, electric cableinsulated with an elastomer coating which can be cross-linked in thepresence of moisture can be produced by carrying out the cross-linkingin an enclosure containing pressurized steam at a temperature betweenabout 150° and 200° C. A method of the same type is also described inFrench Published Application No. 2,297,713.

These prior art methods generally include three steps: during the firststep, the liquid silane is mixed with the polymer or copolymer formingthe main component of the material to be extruded; the other additivesare not combined with this mixture until the extrusion step, which isfollowed by the cross-linking step.

Certain drawbacks of these prior art methods could be eliminated by atechnique described particularly in U.S. Pat. No. 4,117,195, accordingto which the three-step method described above is replaced by a two-stepmethod. Non-copolymerized polyethylene and the necessary additives areeach added separately at the inlet of a screw extruder in which themixing, kneading, and homogenization of the substances are activelycarried out in such a way as to produce the material to be extrudeddirectly at the outlet of the extruder.

The second step of this method is the cross-linking of the shapedmaterial, which constitutes the insulating covering of a cable or apipe. According to U.S. Pat. No. 4,117,195, cross-linking is effected bymeans of a treatment in boiling water, for example.

The present invention is intended to improve still further the methoddescribed in the aforementioned U.S. patent, for it has been noted thatthe manufacture of electric cable or of pipes can be appreciably speededup by reducing the number of steps of the method to two. However, thenecessity of a cross-linking treatment lasting several hours, in a vatof either boiling water or water at a temperature on the order of 95°C., is all the more burdensome in that the first step is so much faster.As a matter of fact, the investment necessary for installing andmaintaining vats containing water at the required temperature preventsproduction costs from being reduced as much as desired.

It is possible, of course, to carry out the cross-linking in a moistatmosphere at room temperature, but this takes three weeks or longer.

It is an object of this invention to provide an improved method of theaforementioned type in which cross-linking may be carried out in a moistatmosphere in only a few days.

To this end, there is provided according to the present invention amethod of producing a tubular element of cross-linked plastic material,particularly an insulating covering for an electric conductor,comprising feeding a screw-extruder divided into a heating zone, aplasticizing zone, and a homogenizing zone, with a polymer capable ofbeing cross-linked by the use of a hydrolyzable unsaturated silane, andwith additives in metered quantities, particularly the hydrolyzableunsaturated silane, the latter being introduced separately into theextruder, a free-radical generator, and a silanol condensation catalyst,preparation of a homogeneous mass by active stirring of the foregoingsubstances in the extruder, extrusion of the homogenized material, andcross-linking of the extruded product in the presence of moisture,wherein the additives include an agent capable of reducing the rate ofcrystallization of the polymer, and cross-linking is carried out in amoist air atmosphere.

There is further provided according to this invention a tubular elementproduced by the foregoing method.

The invention is based upon the unexpected finding that certain knownadditives, which may be incorporated in the polymer in meteredquantities, act to reduce the rate of crystallization of the polymer andgive the extruded product sufficient permeability to water so that thehumidity in the atmosphere allows cross-linking to develop at a ratesuch that the cable or pipe meets the usual specifications after arelatively very short storage time provided that the ambient temperatureis 20° C. or higher and the relative humidity is at least 60%.

Thus, for example, it is possible to insulate electric cable or toproduce cross-linked plastic pipes or tubing by this method in countrieshaving a hot, damp climate by outdoor storage of the reels wound afterthe extrusion step. It has been found that under the conditions statedabove, the rate of cross-linking is such that the cable meets the IEC(International Electrotechnical Commission) specifications after 96 hrs.of storage. If it is necessary to meet even stricter specifications asrequired in certain countries, the storage time must be increasedaccordingly; but it has been found that 12 days of storage suffices tomeet the strictest requirements.

A preferred embodiment of the invention will now be described in detailbased upon the following example.

An electric cable made of copper wires and intended to conduct currentat a voltage of 1 kV was provided with an insulating coating by passingit through an extrusion head fixed to the downstream end of an extruderbarrel. The extruder was one comprising three zones, viz., a heatingzone, a plasticizing zone, and a homogenizing zone, e.g., an extrudersuch as is described in U.S. Pat. No. 4,117,195. Extruders of that typeare also described in GB Pat. No. 964,428 and in U.S. Pat. No.3,358,327. The screw of the extruder included several zones withdiffering thread configurations. In the plasticizing zone, the screw hadtwo interposed threads of different pitches defining two channels, one achannel of decreasing cross-section from which the material is forcedover one of the threads into the second channel of gradually increasingcross-section. This arrangement produced a completely homogeneousmixture of the various materials fed to the hopper of the extruder,e.g., components in the form of powder or pellets and liquid components.These materials comprised polyethylene and various additives,particularly hydrolyzable unsaturated silane, a free-radical generator,and a silanol condensation catalyst, these additives being provided insufficient proportions to bring about the subsequent cross-linking ofthe polyethylene in the presence of water.

In the present example, polyethylene having particles comprising vinylacetate in a proportion of 6% by weight for reducing the crystallizationrate was first produced. The following table indicates the percentagesby weight, relative to the copolymer, of the other substances fed to theextruder:

Carbon black: 10.0%

Dicumyl peroxide: 0.1%

Vinyl trimethoxy silane: 1.5%

Polymerized trimethyl dihydro-quinoline: 0.5%

Dibutyl tin dilaurate: 0.05%

After processing in the extruder, these materials were extruded in atransverse head through which the electric conductor passed.

After extrusion, which took place under normal conditions, and aftercooling of the cable by passing it through a vat of water, the cable waswound on standard reels and stored outdoors in an atmosphere having arelative humidity of 60% and an ambient temperature of 20° C.

After 96 hours of storage, cross-linking was found to have taken place,the rate of cross-linking of the insulation corresponding to the IECspecifications for low-voltage cable.

The proportion of vinyl acetate in the mixture fed to the extruder mayvary within rather wide limits, e.g., from 0.5% to 30%. However, it ispreferable not to exceed 15%, and the most advantageous proportions arefrom 2% to 10%.

Instead of polyethylene, other polyolefins may be used as the polymer,as stated in the aforementioned U.S. Pat. No. 4,117,195.

Instead of vinyl acetate, it is also possible to use polyvinyl acetates,vinyl acrylate, butyl acrylate, or polymers of these substances.

These substances act upon the polymer to lower its rate ofcrystallization during cooling. However, within the aforementionedlimits, they have no unfavorable influence on the dielectric andmechanical quanlities of the insulation, which therefore completelysatisfies the insulating requirements. As for mechanical strength, ithas even been found that the breaking-strength under stress is increasedwhen the crystallization inhibitor is polyvinyl acetate.

Because the crystallization process is retarded, water penetrates deeplyinto the insulating coating even if it is thick. The reaction of thesilane with the polyolefin and with the water therefore takes placedeeply more quickly than when crystallization proceeds normally.

Instead of introducing the agent for reducing the rate ofcrystallization into the hopper of the extruder separately orincorporated in the polymer, it may be introduced by injection into thebarrel of the extruder through a pipe-line connected to an intakeopening in the barrel as taught in U.S. Pat. No. 4,135,825.

What is claimed is:
 1. A tubular element of cross-linked plasticmaterial, particularly an insulating covering for an electric conductor,produced by the method wherein a polymer capable of being cross-linkedby the use of a hydrolyzable unsaturated silane is fed, together withmetered quantities of additives including a free-radical generator and asilanol condensation catalyst, to a screw-type extruder divided into aheating zone, a plasticizing zone, and a homogenizing zone; a meteredquantity of a hydrolyzable unsaturated silane is added separately to theextruder; the foregoing substances are actively stirred in the extruderuntil a homogeneous mass is obtained; the homogenized mass is extruded;and cross-linking of the product is carried out in the presence ofmoisture, whereinamong said additives is an agent capable of reducingthe rate of crystallization of said polymer to enhance penetration ofsaid moisture into said extruded mass, thus increasing the speed of saidcross-linking, said agent drawn from the group consisting of vinylacetate, vinyl acrylate, butyl acrylate and polymers thereof in aproportion of 0.5% to 15% by weight relative to said polymer capable ofbeing cross-linked by the use of a hydrolyzable unsaturated silane, andcarrying out said cross-linking in a moist air atmosphere, said moistair atmosphere exhibiting a relative humidity of at least 60% and atemperature of at least 20° C.